A fuel cell is a device which, by electrochemically oxidizing hydrogen, methanol and the like in the cell, converts the chemical energy of the fuel directly into electric energy and takes out the energy; fuel cells are attracting attention as a clean electric energy supply source. In particular, solid polymer type fuel cells are expected as an alternative source of power for vehicles, a household cogeneration system, and an electric power generator for cellular phones since they operate at low temperatures as compared to other type cells.
Such a solid polymer type fuel cell comprises a membrane/electrode assembly (hereinafter referred to as MEA) in which a pair of catalyst layers are bonded to both surfaces of a proton exchange polymer membrane. More specifically, an anode catalyst layer is provided on one surface of the proton exchange polymer membrane, and a cathode catalyst layer is provided on the other surface of the same membrane. Additionally, an assembly having a structure in which further a pair of gas diffusion layers placed respectively on both outside surfaces of the catalyst layers is also referred to as an MEA.
The anode and cathode catalyst layers have hitherto been sheets made of a mixture comprising a carbon black powder made to support catalyst particles and a proton conductive polymer; among others, in a preparation method, the catalyst layers are bonded to the proton exchange polymer membrane by heat pressing. Incidentally, an electrode assembly having a structure in which such catalyst layers are laminated with gas diffusion layers is generally referred to as a gas diffusion electrode.
A fuel (for example, hydrogen) is supplied to a gas diffusion electrode as the anode, an oxidant (for example, oxygen and air) is supplied to a gas diffusion electrode as the cathode, and a fuel cell comes into operation when both electrodes are connected to an external circuit. Specifically, when hydrogen is used as fuel, hydrogens are oxidized on the anode catalyst to produce protons. The protons thus produced pass through the proton conductive polymer portion in the anode catalyst layer, thereafter migrate through the proton exchange polymer membrane, then pass through the proton conductive polymer portion in the cathode catalyst layer, and thus get onto the cathode catalyst. On the other hand, the electrons generated concurrently with the protons by the oxidation of hydrogens pass through the external circuit and reach the cathode gas diffusion electrode, where the electrons react with the above described protons and the oxygen in an oxidant to generate water, where electric energy can be taken out.
The electric power generation performance of a fuel cell largely depends on the water content regulation in the proton exchange polymer membrane, and in the gas diffusion electrodes of the anode and cathode. Specifically, when the proton exchange polymer membrane is dehumidified, the proton conductance thereof is remarkably decreased and the internal resistance of the cell is increased, resulting in a degradation of the electric power generation performance.
Additionally, when the proton conductive polymer portions constituting the gas diffusion electrodes of the anode and cathode are dehumidified, the internal resistances of the gas diffusion electrodes are increased, and concurrently the activation overvoltage is increased, resulting in a degradation of the electric power generation performance. In particular, in the anode section, when the protons migrate from the anode section to the cathode section through the proton exchange polymer membrane, the protons are accompanied by water molecules, so that the water content of the anode section becomes deficient. Consequently, the proton conductive polymer portion in the anode section tends to be dehumidified, and accordingly the proton migration is suppressed to form a water concentration gradient within the proton exchange polymer membrane and thus the decrease of the proton conductance takes place.
Although, on the other hand, from the viewpoint of simplification of the fuel cell system, it is preferable to operate the fuel cell under the condition of low humidification as possible. As described above there has been the problem that no satisfactory electric power generation performance can be attained unless the proton exchange polymer membrane and the gas diffusion electrodes in the anode and cathode sections are sufficiently humidified.
For the purpose of overcoming the above described problem, JP-A-06-111827 proposes a method in which particulate and/or fibrous silica is contained as a water absorbing material in the anode catalyst layer and/or the cathode catalyst layer (hereinafter referred to as the mixing method), JP-A-06-111834 proposes a method in which particulate and/or fibrous silica is contained in the proton exchange polymer membrane and JP-A-07-326361 proposes a catalyst layer formed by use of a water absorbing material. By use of these techniques, the water holding capacities of the proton exchange polymer membrane and the gas diffusion electrodes in the anode and cathode sections can be increased, and accordingly the water content control can be made easier to some extent.
However, such particulate or fibrous water absorbing material serves to improve the water holding capacity, but at the same time causes the increase in electric resistance and the decrease in gas permeability; thus there is a limit in the effect brought about by such a material. There is an additional problem that when the amount of such a material is increased, the catalyst layers and the proton exchange polymer membrane become brittle, and the catalyst layers cannot bond to the proton exchange polymer membrane. Accordingly, from a practical standpoint, the water content cannot be said to become sufficiently easy to control, such that even when a fuel cell is operated under a condition of low humidification, the effect brought about by such a material has been found to be small (see Comparative Examples 2 to 5).
Incidentally, there has been reported a composite membrane (hereinafter referred to as a sol-gel membrane) in which silica is contained in a perfluorocarbon based ion exchange membrane by taking advantage of the sol-gel reaction. Specifically, a perfluorocarbon based ion exchange membrane is soaked and swollen in an aqueous solution of an alcohol, such as methanol, and thereafter a mixed solvent comprising a tetraethoxysilane (which is a metal alkoxide) and an alcohol is added, and the tetraethoxysilane is subjected to hydrolysis/polycondensation reactions with the aid of the catalytic action of the acidic group. Thus, silica is produced uniformly in the ion exchange membrane (K. A. Mauritz, R. F. Storey and C. K. Jones, in Multiphase Polymer Materials: Blends and Ionomers, L. A. Utracki and R. A. Weiss, Editors, ACS Symposium Series No. 395, P. 401, American Chemical Society, Washington, D.C. (1989)).
However, it has been reported that the water holding capacity under a low humidification condition is only slightly improved, even with an increased amount of silica incorporated. Further, the effect of improving the water holding capacity is small and additionally the proton conductance is decreased (N. Miyake, J. S. Wainright, and R. F. Savinell, Journal of the Electrochemical Society, 148(8), A898-904 (2001)). Accordingly, even when a fuel cell is operated under a low humidification condition, the effect brought about by the sol-gel membrane has been found to be small (see Comparative Example 6).